When an energy source which comprises a number of individual sources, such as an array of air guns, is used in a seismic marine survey, it is an important requirement that the firing times of the individual guns should be known, so that they may be suitably synchronised or allowances made for any differences in the individual firing times. Synchronisation may be effected by means of an electronic control unit, such as that described in U.S. Pat. No. 3,985,199.
In the case of marine air guns attempts have been made to obtain a "gun-operate" signal from the current supplied to a solenoid, which is normally mounted in or on the gun so that energisation of the solenoid causes a valve in the gun to open and allow a blast of compressed air to be released into the surrounding water to produce the pulse. Assumptions have been made that the onset of the current to the solenoid, or a secondary pulse which is obtained from the actual movement of the iron slug in the solenoid, bears constant time relationship with the accoustic time pulse which is generated by the blast of air discharged from the gun. In practice, however it has been found that there is a variable time delay between the current pulse and the air blast, which may amount to several milliseconds. Such variable time delays may occur between the operation of the solenoid and the operation of the valve which releases the air from the gun or they may be caused by the variations in the air pressure in the gun, or by mechanical wear in the moving parts. Whatever the cause they can seriously impair the efficiency and accuracy of the multiple gun array.
The present invention sets out to overcome this problem and to avoid or reduce any errors, which it does by using the actual blast of air discharged into the medium to produce a "gun operate" signal, that is a signal which indicates the time of firing of the gun and the generation of the seismic pulse. It is a valuable feature of this invention that it uses a variation in the resistance of an electrical circuit caused by the air itself to produce the seismic signal or pulse without the intervention of any mechanical switch or other moving parts.
An air gun which is well-known as the energy source in the seismic survey work conducted at sea or in rivers or swamps comprises a casing having a chamber provided with an inlet which is connected to a source of air under pressure. This chamber opens into the surrounding water through several (usually four) large outlet ports formed in its side wall, which ports are normally closed by means of a shuttle valve operating in the casing. With this valve closed, the air pressure is built up in the gun to a pressure of about 2,000 pounds per square inch, the effect of which is to keep the valve closed until a solenoid-operated secondary valve is opened to allow the pressure to act on the main shuttle valve in a direction to cause it to open and allow the rapid discharge of the compressed air from the chamber into the surrounding water. This generates a seismic pulse as the air bubble produced expands into the water.